The present invention relates generally to a method and apparatus for minima enlargement.
Wireless telecommunication systems sometimes are divided into a series of cell areas covering a service area. Each cell area has a transmitting base station using an operating frequency set comprising a plurality of radio channels to communicate with mobile subscribers. Each channel represents an information signal at a particular frequency carrier or band.
In many instances it is advantageous to combine these channels for transmission purposes. The channels can all be combined by a broadband signal combiner into a multi-subchannel signal at lower power levels and then amplified by a single linear amplifier (or its equivalent, a plurality of linear amplifiers in parallel, each amplifying a reduced power version of the same multi-carrier sub-channel signal) to raise the multi-subchannel signal to an appropriate transmit power level.
As data rate requirements rise, the symbol rate necessary in forthcoming protocols will cause the modulation bandwidth to exceed the coherence bandwidth of the channel. This requires an expensive equalizer at the receiver to compensate for intersymbol interference created by the time dispersion in a multipath channel. Another application of multi-subchannel technology is to split the single, high symbol rate modulation into a plurality of lower rate sub-channels that each has a low enough modulation bandwidth to avoid the need for an adaptive equalizer.
Highly linear multi-subchannel modulations have large dynamic ranges where the minimum voltage can approach xe2x88x9250 dB or lower from the signal mean. This will essentially take the linear amplifier used to increase the power of the signal prior to transmission down to idle bias current and cause a spike in the load impedance with unpredictable results. In addition, with recent supply modulation techniques used to increase the linear amplifier efficiency, if the supply voltage approaches zero, the linear amplifier could have problems with phase, noise and stability.
In the past, floor clamp circuits have been employed to prevent the voltage from dropping below a specified threshold. This results in frequency domain splatter and a reduction in linearity of the signal. Furthermore, different types of signals can tolerate different amounts of distortion and thus different amounts of compression. Prior-art techniques have not attempted to tailor symbols on multiple subchannels, including empty subchannels, in a controlled, individualized manner to improve the minima to average power ratio.
Clearly then, a need exists for an improved method and apparatus for minima enlargement.